Right on the heels of Equifax's enormous data breach, which affected up to 143 million people, a major-fast food chain's payment system has been compromised. If you've eaten at Sonic lately, you could be one of the 5 million people at risk.

Sonic Drive-In acknowledged the potential breach to Krebs on Security this week after the website came across a sale of millions of stolen credit and debit accounts. It's not yet clear if all of the 5 million accounts listed for sale on credit card theft site "Joker's Stash" belong to Sonic customers, but a common link in the fraudulent charges already reported to financial institutions was a recent purchase from Sonic.

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Sonic told Krebs they are investigating a "potential incident" at some Sonic locations. In a statement, the company said: "Our credit card processor informed us last week of unusual activity regarding credit cards used at SONIC. The security of our guests' information is very important to SONIC. We are working to understand the nature and scope of this issue, as we know how important this is to our guests."

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The fast-food chain said they have gotten third-party forensic experts and law enforcement involved, and would share more information as they are able to within the limits of the law.

The cards that are for sale are organized by location, so purchasers can buy one from the area they reside. This makes it easier to use the stolen card without triggering bank safety mechanisms that take note of out-of-state charges, and especially important that you check for unusual card activity if you think your card might have been compromised.

According to Krebs, there are some indications that other food chains could have been compromised by the same attackers, though it's not known which those are yet. Regardless, you should keep an eye on your accounts and report any suspicious activity.